1. Technical Field
The invention relates to fuel assemblies for a nuclear reactor cooled and moderated by an upward coolant flow and is particularly, although not exclusively, suitable for use in light water cooled and moderated reactors.
2. Prior Art
A nuclear reactor core is generally formed of fuel assemblies, of hexagonal or square cross-section, supported by a core support plate.
Most presently used fuel assemblies are of the type comprising a bundle of fuel rods (this term designating loaded with fissile material and/or fertile material) and a structure for holding the bundle in position, comprising upper and lower end pieces or nozzles joined together by guide tubes which support grids for maintaining the fuel rods at the nodal points of a regular array; most of them comprise resilient means for transmitting the force impressed on the assembly by the upward coolant flow.
The upper end piece fulfils several mechanical and hydraulic functions. It makes it possible to handle the assembly. It cooperates with an upper core plate which defines the core for fixing the level of the assembly. The upper end piece is also formed with openings for elongated elements insertable in guide tubes and forming clusters; these clusters may be placed permanently (plugs or neutron source clusters) or they may be movable during operation (neutron absorbing or spectrum variation clusters). Substantially the entire upward flow through the assembly between the fuel rods and possibly within and along the guide tubes is collected by the upper end piece.
The upper core plate placed above the assemblies has openings for discharging the coolant. The upper core plate constitutes, with cluster guide sleeves and spacer columns connected to an additional plate, upper internals of the reactor.
In present light water cooled and moderated reactors, the coolant outlet openings formed in the upper end piece are so arranged that substantially the entire flow leaving an assembly receiving a cluster is directed toward the guide sleeve of the cluster. Such arrangements are disclosed in French No. 2,000,399 (Westinghouse), corresponding to U.S. Pat. No. 4,231,843, and in European No. 0,079,828 (Framatome et Cie), to which reference may be made for a general description of the assembly.
In some cases, in this arrangement the flow rate within the cluster guide sleeves is so large or the coolant speed is so high that it induces intense vibrations of the rods of the clusters.